Why does TurboTax post my estimated state taxes for 2019 as Federal income?

The question of whether your state tax refund is taxable depends first on whether you Itemized or took the Standard Deduction in the previous year. The second question is whether you deducted the Sales Tax taken or the State and Local Income Tax (most take the State and Local Income Tax unless they live in a tax-free state). 

You cannot get a deduction on your state income taxes and later receive a tax-free refund. You must adjust your refund to account for the deduction you previously claimed. 

If you Itemized Deduction and took the State and Local Income tax on your Schedule A, then your State Refund is taxable to the extent that you received a benefit from it. 

You can find more information here.